1. Field of the Invention
The present invention relates to a method of preventing deposition of sludge in a liquid tank such as a crude oil tank, a storage tank for rubber or the like, or a water treatment sludge tank, and of removing deposited sludge.
2. Description of the Prior Art
When crude oil is stored in a tank for a long period of time, a clay-like sludge consisting of wax, asphaltene, debris, or water in crude oil, deposits on the bottom of the tank. Although the properties and amounts of such sludge differ in accordance with the type of oil, storage time or method of storage, sludge frequently deposits to a height of 5 m. When such sludge is left unremoved, the oil storage capacity is lowered. In addition, various problems occur such as inclining of a floating roof or constriction of an oil flow path. For these reasons, sludge must be periodically removed or its deposition must be prevented.
Various conventional methods for preventing deposition of sludge in a crude oil tank are known. According to one method, an agitator is installed inside a tank. The agitator is used to agitate the crude oil to impart a flow thereto, thereby preventing deposition of sludge. However, with recent trends toward larger tanks and import of low-quality crude oil, the above-mentioned method cannot satisfactorily prevent deposition of sludge within the entire tank. This method can provide only an agitation effect near the agitator.
In view of this problem, the coapplicant of this application earlier proposed a technique as per Japanese Utility Model Disclosure No. 51-36119 (to be referred to as a basic technique hereinafter). According to this technique, an upright nozzle is arranged at a given position within a crude oil tank. A spray device is mounted on the upright nozzle so as to rotate or pivot while spraying a liquid (oil). The spray device is driven by utilizing a hydraulic pressure obtained when the crude oil in the tank circulates or when oil is externally supplied, thereby constantly spraying oil around the upright nozzle and agitating the crude oil inside the tank.
According to this basic technique, oil sprayed from the nozzle allows agitation of crude oil within the entire tank. This technique can resolve problems that arise with conventional techniques. In addition, according to the basic technique, the sprayed oil is streamed against the sludge deposition layer on the bottom of the crude oil tank so as to break up the deposited sludge and allow it to be redissolved in the crude oil.
However, in order for sludge deposited on the bottom of the crude oil tank to be broken up and agitated by oil sprayed from a nozzle, considerably high pressure and spray energy are required. However, the spray capacity of a spray device is limited. In addition, since various accessories including a roof drain pipe are installed inside the tank, few spray devices can be installed without technical difficulty and at economical cost.
Even if oil sprayed from the nozzle has energy capable of breaking up the sludge, the sludge may not actually be broken up or agitated depending upon the installation position of the spray device and the tank structure.
Furthermore, depending upon the installation position of the spray device in the tank, the sprayed oil acts on the tank side wall or the like, generating noise or causing vibration.
In order to obtain a good agitation effect with sprayed oil within the entire tank while resolving the above problems, the installation positions, spacing and installation method of the spray devices must be properly selected. However, many technical considerations are involved in resolving these problems. Even though almost ten years have passed since the advent of the basic technique, a practical technique which provides a solution to all the problems has not yet been proposed.
For this reason, even if sludge deposited in a tank can theoretically be removed by the basic technique, until now when sludge is actually deposited in the tank, it has been removed manually after draining the tank.
However, the manual method of removing sludge after draining the tank requires much labor and time, and the process is very dangerous.